Help!!! What Size Do I Need To Make?

I am going to be making a cake for 35-40 people. They want it to look similar to the hibiscus cake I did for my sister. I have been wanting to do a stacked/tiered cake for a while and this is my opportunity. My question is what sizes do I need to make? Also should I use a plate set or will dowels and cake boards be ok? I was thinking a 10 an 6 inch . I am excited about this!